Feynman x

  The variable was introduced by Feynman [Feynman69] scaling variable in the discussion of inclusive hadronic interactions at large energies. is the longitudinal momentum of a particle, is the maximum allowed , and , the total centre-of-mass energy of the interaction. Also used sometimes is the transverse x defined by (with the transverse momentum).

In hadron-hadron collisions, x₁ and x₂ are often used to designate the energy fraction carried by the two colliding partons. The parton-parton system is defined (assuming massless partons and head-on collisions) by a centre-of-mass energy The probability density function of the x_i is called the structure function.